Nanopatterned articles produced using surface-reconstructed block copolymer films

ABSTRACT

Nanopatterned surfaces are prepared by a method that includes forming a block copolymer film on a substrate, annealing and surface reconstructing the block copolymer film to create an array of cylindrical voids, depositing a metal on the surface-reconstructed block copolymer film, and heating the metal-coated block copolymer film to redistribute at least some of the metal into the cylindrical voids. When very thin metal layers and low heating temperatures are used, metal nanodots can be formed. When thicker metal layers and higher heating temperatures are used, the resulting metal structure includes nanoring-shaped voids. The nanopatterned surfaces can be transferred to the underlying substrates via etching, or used to prepare nanodot- or nanoring-decorated substrate surfaces.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.12/566,705 filed Sep. 25, 2009, issued on Aug. 27, 2013 as U.S. Pat. No.8,518,837, which claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/100,004 filed Sep. 25, 2008. The priorityapplications are fully incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

The U.S. Government has certain rights in this invention pursuant toNational Science Foundation MRSEC on Polymers Grant No. DMR-0213695, andDepartment of Energy, Office of Basic Energy Sciences Grant No.DE-FG02-96ER45612.

BACKGROUND OF THE INVENTION

Microphase separated block copolymers (BCPs) offer unique opportunitiesto control the spatial distribution of nanoparticles, opening pathwaysto improve the mechanical strength, conductivity, permeability,catalytic activity, and optical and magnetic properties of thin filmsSee, for example, Jaramillo, T. F.; Baeck, S. H.; Cuenya, B. R.;McFarland, E. W. J. Am. Chem. Soc. 2003, 125, 7148; Feldheim, D. L.;Grabar, K. C.; Natan, M. J.; Mallouk, T. E. J. Am. Chem. Soc. 1996, 118,7640; Honda, K.; Rao, T. N.; Tryk, D. A.; Fujishima, A.; Watanabe, M.;Yasui, K.; Masuda, H. J. Electrochem. Soc. 2001, 148, A668; Bockstaller,M. R.; Mickiewicz, R. A.; Thomas, E. L. Adv. Mater. 2005, 17, 1331; andFan, H. J.; Werner, P.; Zacharias, M. Small 2006, 2, 700. The ability tocontrol the orientation and lateral ordering of BCP morphologies makesBCPs particularly attractive as scaffolds and templates for thefabrication of nanostructured materials. See, for example, Haryono, A.;Binder, W. H. Small 2006, 2, 600; Black, C. T.; Murray, C. B.;Sandstrom, R. L.; Sun, S. Science 2000, 290, 1131; Thurn-Albrecht, T.;Schotter, J.; Kastle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum,L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science2000, 290, 2126; and Lopes, W. A.; Jaeger, H. M. Nature 2001, 414, 735.Several methods for incorporating inorganic nanoparticles into polymericnanostructures have been described. In one, nanoparticles are generatedwithin block copolymer micelles, where metal nanoparticles can beproduced by simple chemical methods. See, for example, Cheng, G.;Moskovits, M. Adv. Mater. 2002, 14, 1567; Gorzolnik, B.; Mela, P.;Moeller, M. Nanotechnology 2006, 17, 5027; and Sohn, B.-H.; Choi, J.-M.;Yoo, S. I.; Yun, S.-H.; Zin, W.-C.; Jung, J. C.; Kanehara, M.; Hirata,T.; Teranishi, T. J. Am. Soc. Chem. 2003, 125, 6368. In another, thecooperative self-organization of nanoparticles and block copolymers isused with the need of subsequent chemistry. See, for example, Lopes, W.A.; Jaeger, H. M. Nature 2001, 414, 735; (13) Thompson, R. B.; Ginzburg,V. V.; Matsen, M. W.; Balazs, A. C. Science 2001, 292, 2469; Hamley, I.W. Angew. Chem. Int. Ed. 2003, 42, 1692; Chiu, J. J.; Kim, B. J.;Kramer, E. J.; Pine, D. J. J. Am. Soc. Chem. 2005, 127, 5036; Kim, B.J.; Chiu, J. J.; Yi, G.; Pine, D. J.; Kramer, E. J. Adv. Mater. 2005,17, 2618; Lin, Y.; Böker, A.; He, J.; Sill, K.; Xiang, H.; Abetz, C.;Li, X.; Wang, J.; Emrick, T.; Long, S.; Wang, Q.; Balazs, A.; Russell,T. P. Nature 2005, 434, 55; Ansari, I. A.; Hamley, I. W. J. Mater. Chem.2003, 13, 2412.

The surface reconstruction of BCPs, as reported previously, is anothermethod to this end. See, for example, Xu, T.; Stevens, J.; Villa, J.;Goldbach, J. T.; Guarini, K. W.; Black, C. T.; Hawker, C. J.; Russell,T. P. Adv. Funct. Mater. 2003, 13, 698; Park, S.; Wang, J.-Y.; Kim, B.;Chen, W.; Russell, T. P. Macromolecules 2007, 40, 9059; and Park, S.;Kim, B.; Wang, J.-Y.; Russell, T. P. Adv. Mater. 2008, 20, 681. Surfacereconstruction is a process where, in the case of a diblock copolymerwith cylindrical microdomains oriented normal to the surface, uponexposure of the BCP film to a solvent that preferentially dissolves theminor component block, the minor component is drawn to the surface ofthe film, and, upon drying, cylindrical nanopores are produced withdimensions comparable to the original cylindrical microdomains. Theminor component block fully coats the surface of the nanoporous filmand, as shown by grazing incidence x-ray scattering, if the filmthickness is a period of the BCP or less, the nanopores were found tospan the film and had vertical side walls. Since the solvent does notalter the chemical structure of the BCP, the reconstruction is fullyreversible. So, by heating the film to near its glass-transitiontemperature, T_(g), a full recovery of the initial thin film morphologyoccurs. However, if the BCP film is heated to temperatures well inexcess of T_(g), then interfacial interactions will control theorientation of the microdomains. If, prior to heating, metal isevaporated at a glancing angle onto the surface of the reconstructedfilm, a porous metal film is obtained. In most pattern transferapproaches, a nanoporous polymer template has been used to transfer apattern into underlying substrates using RIE and/or milling (see, forexample, Park, M.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Appl.Phys. Lett. 2001, 79, 257; Cheng, J. Y.; Ross, C. A.; Thomas, E. L.;Smith, H. I.; Vancso, G. J. Appl. Phys. Lett. 2002, 81, 3657; Meli,M.-V.; Badia, A.; Grütter, P.; Lennox, R. B. Nano. Lett. 2002, 2, 131);Guarini, K. W.; Black, C. T.; Zhang, Y.; Kim, H.; Sikorski, E. M.;Babich, I. V. J. Vac. Sci. Technol. B 2002, 20, 2788; and Kubo, T.;Parker, J. S.; Hillmyer, M. A.; Leighton, C. Appl. Phys. Lett. 2007, 90,233113), while the control of spatial location of metal on polymertemplate can be used as etching masks for preparation of various kindsof nanostructured patterns.

BRIEF DESCRIPTION OF THE INVENTION

Here we demonstrate the generation of various nanopatterned surfacesthat can be obtained from the reconstructed BCP film coated with a thinlayer of metal. Thin block copolymer films having cylindricalmicrodomains oriented normal to the surface were used. Reconstruction ofthe block copolymer films, followed by glancing angle metal evaporationonto the surface heating of the multilayer structure thus obtained ledto various metal masks or templates that were suitable for patterntransfer into substrates using etching, or as templates for furthermetal evaporation to generate nanoscopic metal rings.

One embodiment is a method of preparing a nanopatterned surface,comprising: forming a block copolymer film on a substrate; wherein theblock copolymer film has a thickness and comprises a major phase and aminor phase; and wherein the minor phase comprises cylindrical domainsextending through the thickness of the block copolymer film; annealingthe block copolymer film in an annealing solvent vapor to form anannealed block copolymer film; surface reconstructing the annealed blockcopolymer film to form a surface-reconstructed block copolymer film;wherein the surface-reconstructed block copolymer film comprises a majorphase layer comprising the major phase and being in contact with asurface of the substrate, and a minor phase layer comprising the minorphase and being in contact with a surface of the major phase layeropposite the substrate; and wherein the surface-reconstructed blockcopolymer film defines an array of cylindrical pores; depositing a metalon the surface reconstructed block copolymer film to form a metal-coatedblock copolymer film comprising a metal layer contacting the minor phaselayer on a surface of the minor phase layer opposite the major phaselayer; and heating the metal-coated block copolymer film to form aredistributed metal-coated block copolymer film in which at least aportion of the metal has been transferred into the cylindrical pores.

Another embodiment is a nanopatterned article, comprising: a substrate;and a metal-decorated block copolymer film in contact with a surface ofthe substrate; wherein the metal-decorated block copolymer film has athickness and comprises a continuous phase and a dispersed phasecomprising cylindrical domains extending through the thickness of thefilm; wherein the continuous phase comprises a major phase of the blockcopolymer; wherein the dispersed phase comprises a minor phase of theblock copolymer and nanodots comprising metal; and wherein the nanodotsin the dispersed phase constitute at least 50 percent of the metalcontent of the article.

Another embodiment is a nanopatterned article, comprising: a substrate;a metal-decorated block copolymer film in contact with a surface of thesubstrate; and a metal layer in contact with a surface of themetal-decorated block copolymer film opposite the substrate; wherein themetal-decorated block copolymer film comprises a block copolymer andmetal nanodots; and wherein the metal layer defines an array ofessentially circular voids, each circular void disposed above a nanodot.

These and other embodiments are described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 consists of scanning force microscope (SFM) images ofwell-developed cylindrical micro domains and the corresponding structureafter surface reconstruction; the three images show the block copolymerfilm (a) after solvent annealing, (b) after surface reconstruction, and(c) after thermal annealing.

FIG. 2 shows TEM images of three different reconstructed PS-b-P4VP filmsdecorated with gold in different ways, depending on the experimentalconditions. FIG. 2(a) shows gold nanoparticles on top of a solventreconstructed film; FIG. 2(b) shows the product of heating a film likethat of FIG. 2(a) at 115° C. for 10 minutes; FIG. 2(c) shows the productof heating a block copolymer film with a thicker gold layer at 180° C.for 30 minutes; insets in each image are magnified images.

FIG. 3 consists of grazing incidence-small angle x-ray scattering(GISAXS) traces taken along the horizon, that is, along q_(y) (q_(z)=0),for gold-coated, solvent-reconstructed block copolymer films; (a, b) topof the reconstructed films, (c,d) inside the poly(4-vinylpyridine)cylindrical domains, and (e,f) top of films and insidepoly(4-vinylpyridine) cylindrical domains; for images (a)-(c), α=0.12(below the critical angle of the polymer), and for images (d)-(f),α=0.18 (above the critical angle of the polymer).

FIG. 4 shows pattern transfer of three gold-decorated polymer templatesinto silicon oxide using SF₆ reactive ion etching; in the inset, patterntransfer results are magnified for clarity.

FIG. 5 shows the SEM image of a chromium nanoring pattern obtained bycoating chromium onto gold-decorated films having nanoring-shaped voids;in the magnified inset in the top-left corner of FIG. 5, a chromiumnanoring pattern with a long-range order is clearly seen.

FIG. 6 illustrates the process of gold decorated films prepared fromnanoporous BCP templates.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment is a method of preparing a nanopatterned surface,comprising: forming a block copolymer film on a substrate; wherein theblock copolymer film has a thickness and comprises a major phase and aminor phase; and wherein the minor phase comprises cylindrical domainsextending through the thickness of the block copolymer film; annealingthe block copolymer film in an annealing solvent vapor to form anannealed block copolymer film; surface reconstructing the annealed blockcopolymer film to form a surface-reconstructed block copolymer film;wherein the surface-reconstructed block copolymer film comprises a majorphase layer comprising the major phase and being in contact with asurface of the substrate, and a minor phase layer comprising the minorphase and being in contact with a surface of the major phase layeropposite the substrate; and wherein the surface-reconstructed blockcopolymer film defines an array of cylindrical pores; depositing a metalon the surface reconstructed block copolymer film to form a metal-coatedblock copolymer film comprising a metal layer contacting the minor phaselayer on a surface of the minor phase layer opposite the major phaselayer; and heating the metal-coated block copolymer film to form aredistributed metal-coated block copolymer film in which at least aportion of the metal has been transferred into the cylindrical pores. Asused herein, the term “nanopatterned” means comprising a pattern offeatures having dimensions of about 5 to about 500 nanometers,specifically about 10 to about 100 nanometers.

The first step of the method is forming a block copolymer film on asubstrate. In general, the substrate may comprise any material that iswettable by the block copolymer and resistant to the solvents used forspin coating and solvent annealing of the block copolymer film. Forembodiments in which reactive ion etching is used to etch theresist-coated block copolymer film, the substrate can comprise amaterial capable of being etched by reactive ion etching. Suitablematerials include, for example, silicon, doped silicon, silicon dioxide,silicon nitride, passivated silicon, polystyrenes, polyimides,poly(butylenes terephthalate)s, and germanium. In some embodiments, thesubstrate surface in contact with the block copolymer film is chemicallyhomogeneous. In some embodiments in which long-range order in thenanopatterned article is desirable, the substrate is a miscut, annealedcrystalline substrate. The formation of block copolymer films on miscut,annealed crystalline substrates is described in Thomas P. Russell etal., U.S. Provisional Patent Application Ser. No. 61/098,253, filed Sep.19, 2008.

The block copolymer can be formed using methods known in the art. Insome embodiments, forming the block copolymer film comprises spincoating the block copolymer film from solution onto the substrate. Forexample, a solution comprising a polystyrene-poly(4-vinylpyridine)diblock copolymer in a mixture of toluene and tetrahydrofuran can bespin coated onto the substrate.

The block copolymer used for film forming comprises a major phase,corresponding to the continuous phase of the film, and a minor phase,corresponding to cylindrical domains extending through the thickness ofthe film. In order to form the two spatially distinct phases, the blockcopolymer should comprise at least two chemically distinct blocks. Thus,the block copolymer comprises a first block and second block that aresufficiently incompatible with each other to form separate phases.Incompatibility of the two blocks can be characterized by a differencein the Hildebrand solubility parameters of the two blocks. For example,when the block copolymer comprises a first block having a firstHildebrand solubility parameter and a second block having a secondHildebrand solubility parameter, the first Hildebrand solubilityparameter and the second Hildebrand solubility parameter can differ byat least 0.4 megapascal^(1/2).

In order to facilitate redistribution of a portion of the metal layerinto the cylindrical voids formed on solvent reconstruction, it ispreferable that at least one block of the block copolymer (i.e., theblock corresponding to the minor phase of the block copolymer film)comprise functional groups capable of favorably interacting with thedeposited metal. For example, at least one block of the copolymer cancomprise basic nitrogen atoms, such as those in a pyridine group.Alternatively, at least one block of the copolymer can comprise basicoxygen atoms, such as the ether oxygen atoms in a poly(ethylene oxide)block.

In some embodiments, the block copolymer comprises apoly(vinyl-substituted nitrogen heterocycle) block that is thepolymerization product of a vinyl-substituted nitrogen heterocycleselected from the group consisting of 2-vinylpyridine, 3-vinylpyridine,4-vinylpyridine, 2-methyl-5-vinylpyridine 1-vinylimidazole,2-vinylimidazole, 4-vinylimidazole, N-vinyl-2-methylimidazole,N-vinyl-2-ethylimidazole, 2-vinylpyrrole, 3-vinylpyrrole, and mixturesthereof. In some embodiments, the block copolymer comprises apoly(4-vinylpyridine) block or a poly(2-vinylpyridine) block.

In some embodiments, the block copolymer comprises a poly(alkyleneoxide) block that is the polymerization product of a C₂-C₆ alkyleneoxide. Examples include poly(ethylene oxide) and poly(propylene oxide).

The polymer block corresponding to the major phase of the blockcopolymer film can vary widely in chemical structure, as long as it isincompatible with the minor phase block. Suitable major phase blocksinclude, for example, polyolefins, poly(alkenyl aromatic)s,poly(conjugated dienes)s, hydrogenated poly(conjugated dienes)s,poly(vinyl-substituted nitrogen heterocycle)s, poly(alkyl(meth)acrylate)s, poly((meth)acrylic acid)s, poly(alkylene oxide)s,poly(arylene oxide)s, poly(arylene sulfide)s, poly(vinyl alkanoates),poly(vinyl ether)s, poly(vinyl halide)s, poly(vinyl alcohol)s,polyurethanes, poly(meth)acrylonitriles, polyesters, polyamides,polyimides, polycarbonates, polysulfones, and polysiloxanes.

In some embodiments, the block copolymer corresponding to the majorphase of the block copolymer film is a poly(alkenyl aromatic) block thatis the polymerization product of an alkenyl aromatic monomer having thestructure

wherein R¹ is hydrogen or C₁-C₆ alkyl, and each occurrence of R², R³,R⁴, R⁵, and R⁶ is independently selected from the group consisting ofhydrogen, C₁-C₆ alkyl, and halogen. In some embodiments, the blockcopolymer corresponding to the major phase of the block copolymer filmis a polystyrene block.

In order to form a film comprising cylindrical domains, the blockcopolymer typically comprises blocks differing in number averagemolecular weight by at least a factor of 1.5. In some embodiments, thenumber average molecular weights of the blocks differ by a factor of 1.5to 6, more specifically a factor of 2 to 5, still more specifically afactor of 3 to 4.

In a very specific embodiment, the block copolymer is apolystyrene-poly(4-vinylpyridine) or polystyrene-poly(2-vinylpyridine)diblock copolymer. In another very specific embodiment, the blockcopolymer is a polystyrene-poly(ethylene oxide) diblock copolymer.

The block copolymer film typically has a thickness of about 10 to about100 nanometers. In some embodiments, the block copolymer film has athickness corresponding to about one period of the block copolymer. Therelationship between film thickness and the period of the blockcopolymer can be determined using methods known in the art. See, forexample, T. P. Russell, P. Lambooy, J. G. Barker, P. D. Gallagher, S. K.Satija, G. J. Kellogg, and A. M. Mayes, Macromolecules 1995, 28, 787;and A. M. Mayes, S. K. Kumar, MRS Bulletin, 1997, 22, 43.

Once the block copolymer film is formed, it is annealed. In someembodiments, the block copolymer film is thermally annealed. Forexample, when the block copolymer is a polystyrene-block-poly(n-butylmethacrylate) diblock copolymer, the block copolymer can be annealed at170° C. for four days. In other embodiments, the block copolymer film isannealed by exposure to solvent vapors. For example, in embodiments,when the block copolymer is a polystyrene-poly(4-vinylpyridine) diblockcopolymer, the annealing solvent vapor can comprise toluene andtetrahydrofuran.

The block copolymer film comprises cylindrical domains extending throughthe thickness of the block copolymer film. In some embodiments, theannealed block copolymer film comprises a hexagonal array of cylindricalmicrodomains. Such hexagonal arrays can exhibit an orientation order ofat least 0.9 over an area of at least 1 centimeter². Ordered arrays inareas of 2.25 centimeter² (1.5 centimeter×1.5 centimeter) aredemonstrated in the working examples below. Areas of at least 25centimeter² (5 centimeter×5 centimeter) are feasible with the techniquesdescribed herein. In contrast, lithographic techniques typicallygenerate ordered arrays over areas on the order of 250 micrometer (50micrometers×50 micrometers). Orientation order can be determined byimage analysis of scanning force microscopy images of the blockcopolymer films using, for example, MATLAB software from. The Mathworks.The cylindrical microdomains can be separated by a nearest-neighbordistance of about 10 to about 100 nanometers.

After the block copolymer film is annealed, it is surface reconstructed.The surface reconstruction method is described in T. Xu, C. J. Hawker,and T. P. Russell, Advanced Functional Materials 2003, 13, 698, and T.P. Russell et al. U.S. patent application Ser. No. 12/049,541, filedMar. 17, 2008. The surface reconstruction method typically consists ofexposing the annealed block copolymer film to a solvent thatpreferentially dissolves the minor phase of the block copolymer film andeffects transfer of that phase to the surface of the film opposite thesubstrate. It is important to note that no chemical bonds are broken inthe surface reconstruction process, and that the process is reversible(for example, by the application of heat). Surface reconstruction isconducted at a temperature below the glass transition temperature of themajor (matrix) phase of the block copolymer film, so that the structureof the film is conserved in that the spaces formerly occupied by theminor phase are converted to voids. Thus, when the minor phase consistsof cylinders perpendicular to the plane of the film, surfacereconstruction results in migration of the minor phase to the topsurface of the film and formation of cylindrical voids where the minorphase formerly resided. The cylindrical voids extend from the substratesurface through the major phase-containing layer and the minorphase-containing layer to the top of the block copolymer film. In someembodiments, the block copolymer is a polystyrene-poly(4-vinylpyridine)diblock copolymer, and surface reconstructing the annealed blockcopolymer film comprises immersing the block copolymer film in a loweralkanol solvent such as methanol, ethanol, 1-propanol, 2-propanol, or amixture thereof.

After the block copolymer film is surface reconstructed, metal isdeposited on the surface-reconstructed film. For simplicity, the term“metal layer” is used herein to describe the form of the depositedmetal. However, particularly at low deposition levels, the depositedmetal need not form a continuous metal layer. It can instead consist ofmetal nanoparticles that do not form a continuous layer.

The thickness of the metal layer will depend on the ultimate structuredesired. In order to form a template in which the metal exists as metalnanodots within the cylindrical domains, the metal layer typically has athickness of about 0.15 to about 0.5 nanometers, specifically 0.2 toabout 0.4 nanometers. These are nominal thickness values, calculated byweighing the metal deposit over a known area of the block copolymer filmand assuming a uniformly thick metal layer. As demonstrated in theworking examples below, particularly at low nominal thicknesses, theactual metal deposit can be discontinuous, with some areas of the filmnot covered in metal and other areas of the film covered with metalnanoparticles of dimensions greater than the nominal metal layerthickness.

In order to form a template in which the metal defines nanoring-shapedvoids, the metal layer typically has a thickness of about 0.5 to about 5nanometers, specifically 1 to 4 nanometers, more specifically 2 to 3nanometers.

There is no particular limitation on the type of metal to be deposited.Suitable metals include, for example, those of Groups 3-12 of theperiodic table and alloys thereof, and specifically gold, silver, andchromium. In order to preserve the cylindrical voids in thesolvent-reconstructed film, metal is typically deposited from a glancingangle relative to the plane of the block copolymer film.

After metal is deposited on the surface-reconstructed block copolymerfilm, the resulting metal-coated block copolymer film is heated to forma redistributed metal-coated block copolymer film in which at least aportion of the metal has been transferred into the cylindrical pores. Insome embodiments, the metal-coated block copolymer film is heated to atemperature within about 40° C. of the glass transition temperature ofthe minor phase block. For example, to form a template in which themetal exists as metal nanodots within the cylindrical domains, ametal-coated block copolymer film comprising a metal layer with athickness of about 0.15 to about 0.5 nanometers can be heated at about105 to about 125° C., specifically about 110 to about 120° C., for about5 to about 20 minutes, specifically about 5 to about 15 minutes. Thisstep results in migration of both the block copolymer minor phase andthe metal (i.e., substantially all of the metal) into the cylindricalvoid. The resulting template thus comprises a block copolymer film withgold-containing cylindrical domains. Experiments described below showthat the metal nanodots are in contact with the substrate.

To form a template in which the metal exists defines nanoring-shapedvoids, a metal-coated block copolymer film comprising a metal layer witha thickness of about 0.5 to about 5 nanometers is heated at about 170 toabout 190° C., specifically about 175 to about 185° C., for about 20 toabout 40 minutes, specifically 25 to 35 minutes. This heating stepredistributes a small portion of the metal layer from ring-shaped areassurrounding the cylindrical pores into the center of the cylindricalpores, thus creating nanoring-shaped voids in metal coverage of thesubstrate, when viewed from above the metal coated film. The portion ofmetal transferred to the cylindrical domains is typically small that theportion that would be transferred from a thinner metal layer in thenanodot method described above. The resulting template withnanoring-shaped voids can be further processed to form metal nanorings.Specifically, the template with nanoring-shaped voids can be furtherprocessed by etching the redistributed metal-coated block copolymer film(to remove polymer surrounding the metal nanoparticles in thecylindrical microdomains), depositing a second metal onto the etched,redistributed metal-coated block copolymer film, thereby formingnanorings comprising the second metal and contacting the substrate, andseparating the substrate and nanorings from the etched, redistributedmetal-coated block copolymer film. The etching step can each beconducted with reactive ion etching methods known in the art. Forexample, the etching step can comprise reactive ion etching with CHF₃,SF₆, or CF₄, or a combination of the foregoing compounds. The secondmetal can be a metal from Groups 3-12 of the periodic table and alloysthereof, and specifically gold, silver, and chromium, more specificallychromium. The second metal can be the same as or different from thefirst metal (i.e., the metal deposited onto the surface-reconstructedblock copolymer film) In some embodiments, the first metal is gold andthe second metal is chromium. Methods described above for forming themetal layer can also be used to deposit the second metal. Separation ofthe nanoring-decorated substrate from the etched, redistributedmetal-coated block copolymer film can be accomplished with methods knownin the art for separating block copolymer films from substrates. Asuitable method demonstrated in the working examples below utilizessonication to effect the separation. Various “lift-off” techniques canalso be employed.

The metal nanodot- and nanoring-containing templates can be furtherprocessed to transfer the metal nanopattern to the substrate. Forexample, the nanodot-containing template can be used to generate asubstrate comprising nanopillars using a method in which the metal layerhas a thickness of about 0.15 to about 0.5 nanometers, the metal coatedblock copolymer film is heated at about 105 to about 125° C. for about 5to about 20 minutes (thereby redistributing the metal layer into thecylindrical pores to form nanodots in contact with the substrate), andthe method further comprises etching through the redistributedmetal-coated block copolymer film and into the substrate (wherein themetal nanodots serve as an etch resist protecting the tops of theincipient pillars). After this process, the only remaining metal is themetal nanodots on the top of the substrate pillars. If desired, thesemetal nanodots can be removed by a technique to selectively dissolve themetal (e.g., use of a potassium iodide/iodine solution). Particularly ifthe metal nanodots are not removed, the structure can, optionally, befurther etched to remove any residual block copolymer. The first(required) etching step can comprise reactive ion etching with SF₆. Thesecond (optional) etching step can comprise oxygen plasma etching. Thismethod is demonstrated in the working examples below.

As another example, the template comprising nanoring-shaped voids can beused to generate a substrate comprising nanoring-shaped voids using amethod in which the metal layer has a thickness of about 0.5 to about 5nanometers, the metal coated block copolymer film is heated at about 170to about 190° C. for about 20 to about 40 minutes thereby redistributinga portion of the metal layer from a ring surrounding the cylindricalpores into the cylindrical pores (thus creating nanoring-shaped voids inmetal coverage of the substrate, when viewed from above the metal coatedfilm), and the method further comprises etching through theredistributed metal-coated block copolymer film and into the substrate,and separating the etched, redistributed metal-coated block copolymerfilm from the etched substrate (but leaving metal caps on the pillarsinside the ring-shaped voids), thereby forming an etched substratecomprising a surface defining nanoring-shaped voids. The structure can,optionally, be further etched to remove any residual block copolymer.The first (required) etching, separating, and second (optional) etchingsteps can be conducted as described in the previous paragraph. Thismethod is demonstrated in the working examples below.

The metal nanodot- and nanoring-containing templates can also be used toform structures in which a conducting or semiconducting layer is formedon top of a substrate with a metal nanodot or nanoring array.

In a very specific embodiment of the method suitable for forming a metalnanodot-decorated article, the block copolymer film comprises apolystyrene-block-poly(4-vinylpyridine) diblock copolymer comprising apolystyrene block having a number average molecular weight of about6,000 to about 30,000 atomic mass units and a poly(4-vinylpyridine) orpoly(2-vinylpyridine) block having a number average molecular weight ofabout 2,000 to about 10,000 atomic mass units, wherein the ratio of thenumber average molecular weight of the polystyrene block to the numberaverage molecular weight of the poly(4-vinylpyridine) orpoly(2-vinylpyridine) block is about 2 to about 6; the annealing theblock copolymer film comprises exposing the block copolymer film to anannealing solvent vapor comprising toluene and tetrahydrofuran; thesurface reconstructing the annealed block copolymer film comprisesimmersing the annealed block copolymer film in a solvent comprising aC₁-C₃ alkanol; the metal comprises gold; the metal layer has a thicknessof about 0.15 to about 0.5 nanometers; and the heating the metal coatedblock copolymer film comprises heating at about 105 to about 125° C. forabout 5 to about 20 minutes, thereby redistributing the minor phase andthe metal layer into the cylindrical pores to form nanodots in contactwith the substrate.

In a very specific embodiment of the method suitable for forming anarticle comprising nanoring-shaped voids, the block copolymer filmcomprises a polystyrene-block-poly(4-vinylpyridine) diblock copolymercomprising a polystyrene block having a number average molecular weightof about 6,000 to about 30,000 atomic mass units and apoly(4-vinylpyridine) or poly(2-vinylpyridine) block having a numberaverage molecular weight of about 2,000 to about 10,000 atomic massunits, wherein the ratio of the number average molecular weight of thepolystyrene block to the number average molecular weight of thepoly(4-vinylpyridine) or poly(2-vinylpyridine) block is about 2 to about6; the annealing the block copolymer film comprises exposing the blockcopolymer film to an annealing solvent vapor comprising toluene andtetrahydrofuran; the surface reconstructing the annealed block copolymerfilm comprises immersing the annealed block copolymer film in a solventcomprising a C₁-C₃ alkanol; the metal comprises gold; the metal layerhas a thickness of about 0.5 to about 5 nanometers; and the heating themetal coated block copolymer film comprises heating at about 170 toabout 190° C. for about 20 to about 40 minutes, thereby redistributing aportion of the metal layer from ring-shaped areas surrounding thecylindrical pores into the cylindrical pores (thus creatingnanoring-shaped voids in metal coverage of the substrate, when viewedfrom above the metal coated film) In order to form a substratecomprising chromium nanorings, the above method can further compriseetching the redistributed metal-coated block copolymer film (to removepolymer surrounding the gold nanoparticles in the cylindricalmicrodomains), depositing a second metal comprising chromium onto theetched, redistributed metal-coated block copolymer film, thereby formingnanorings comprising the second metal and contacting the substrate, andseparating the substrate and nanorings from the etched, redistributedmetal-coated block copolymer film. The separating step can be conductedby sonication or lift-off techniques capable of separating the blockcopolymer film from the substrate.

The invention extends to certain articles prepared by the method andbelieved to be novel independent of their method of preparation. Thus,one embodiment is a nanopatterned article, comprising: a substrate; anda metal-decorated block copolymer film in contact with a surface of thesubstrate; wherein the metal-decorated block copolymer film has athickness and comprises a continuous phase and a dispersed phasecomprising cylindrical domains extending through the thickness of thefilm; wherein the continuous phase comprises a major phase of the blockcopolymer; wherein the dispersed phase comprises a minor phase of theblock copolymer and nanodots comprising metal; and wherein the nanodotsin the dispersed phase constitute at least 50 weight percent of themetal content of the article. In some embodiments, the nanodots in thedispersed phase constitute at least 60 percent of the metal content ofthe article, specifically at least 70 percent of the metal content ofthe article, more specifically at least 80 percent of the metal contentof the article, still more specifically at least 90 weight percent ofthe metal content of the article.

Another embodiment is a nanopatterned article, comprising: a substrate;a metal-decorated block copolymer film in contact with a surface of thesubstrate; and a metal layer in contact with a surface of themetal-decorated block copolymer film opposite the substrate; wherein themetal-decorated block copolymer film comprises a block copolymer andmetal nanodots; and wherein the metal layer defines an array ofessentially circular voids, each circular void disposed above a nanodot.Put differently, where a hypothetical cylinder has a major axisperpendicular to the metal layer and a radius in contact with a circularvoid, and the hypothetical cylinder extends through a circular void anddown to the substrate, the hypothetical cylinder would encompass ananodot. In other words, when viewed from above (i.e., from the metallayer side of the article), a nanodot can be seen in the center of eachcircular void. An example of such a structure is shown in FIG. 2(c).

Another embodiment is a nanopatterned article, comprising: a substrate;and an array of metal nanorings contacting a surface of the substrate;wherein the surface of the substrate has an area of at least 1centimeter²; and wherein the metal nanorings have an outer diameter ofabout 10 to about 50 nanometers. The metal nanoring outer diameter canbe 15 to 40 nanometers, specifically 20 to 35 nanometers. The size ofthe metal nanoring outer diameter can be controlled by varying themolecular weight of the polymer block that forms the cylindricaldomains. The method of fabricating such a nanopatterned article isdescribed above, and a specific embodiment of the nanopatterned articleis fabricated in the working examples below.

Another embodiment is a nanopatterned article, comprising: a substratecomprising a surface defining an array of ring-shaped voids; wherein thesurface has an area of at least 1 centimeter²; and wherein thering-shaped voids have an outer diameter of about 10 to about 50nanometers. The ring-shaped void outer diameter can be 15 to 40nanometers, specifically 20 to 35 nanometers. The size of the outerdiameter of the ring-shaped voids can be controlled by varying themolecular weight of the polymer block that forms the cylindricaldomains. The method of fabricating such a nanopatterned article isdescribed above, and a specific embodiment of the nanopatterned articleis fabricated in the working examples below.

The invention is further illustrated by the following non-limitingexamples.

EXAMPLES

Block copolymer films were prepared by spin-coating 0.5 weight percentpolystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) copolymertoluene/tetrahydrofuran (THF) solutions onto silicon wafers. Thecopolymer used in the experiments was a PS-b-P4VP diblock (obtained fromPolymer Source) with a molecular weight of 68.7 kg/mol (M_(n) ^(PS)=47.6kg/mol; M_(n) ^(P4VP)=20.9 kg/mol) and polydispersity of 1.14. To makehighly oriented cylindrical P4VP microdomains in a PS matrix, the filmswere vapor annealed in a saturated toluene/THF (20/80, volume/volume)environment for 6 hours. Surface reconstruction was achieved byimmersing the films in ethanol for 20 minutes, where ethanol is a goodsolvent for P4VP but a non-solvent for PS. To prepare transmissionelectron microscopy (TEM) samples, the gold-decorated PS-b-P4VP filmswere floated off from the silicon substrate in 0.5 wt % HF solution andcollected on carbon-coated grids. Bright-field TEM was performed on a(JEOL-1200EX) TEM operating at an accelerating voltage of 100 kV.

FIG. 6 illustrates the process of gold decorated films prepared fromnanoporous BCP templates. Highly oriented cylindrical microdomains weredeveloped after solvent annealing in toluene/THF mixed solvent(structure (a)). Nanoporous films were produced by swelling thecylindrical microdomains of the copolymer with a preferential solvent,such as ethanol (structure (b)). Since neither block of the copolymerwas fundamentally altered by the solvent, the process was fullyreversible. By heating the film to near its glass-transitiontemperature, full recovery of the initial film morphology was achieved.Nanoporous gold film was produced by gold sputtering at a glancing angle(about 3-5°) to the substrate without entering into the pores (structure(c)). By varying the thickness of the gold layer and thermal annealingcondition, gold dots (d) and rings (e) were obtained.

FIG. 1 shows scanning force microscope (SFM) images of well-developedcylindrical microdomains and the corresponding structure after surfacereconstruction. Cylindrical microdomains oriented normal to thesubstrate, having hexagonal order immediately after spin coating havebeen shown previously. Park, S.; Wang, J.-Y.; Kim, B.; Chen, W.;Russell, T. P. Macromolecules 2007, 40, 9059. Highly oriented arrays ofcylindrical microdomains with about 2 nanometer depressions wereobtained by solvent annealing and are shown in FIG. 1(a). Areconstructed film having arrays of highly ordered nanopores is shown inFIG. 1(b). The preferential solvation of P4VP blocks with ethanol isshown to produce a nanoporous template while preserving the order ororientation of the microdomains. FIG. 1(c) shows an SFM image of thefilm after annealing the reconstructed film at 115° C. for 10 minutes.It should be noted that the SFM images show that the original structureis retained without any significant change except for changing the filmthickness throughout the entire process, where a hexagonal array ofcylindrical micro domains or nanopores with an average nearest-neighbordistance of 45.3±2.3 nanometers and pore diameter of 25.0±1.7nanometers. The surface roughness after reconstruction was <0.5nanometer.

The reconstructed films are kinetically trapped in an energeticallyunstable state, due to the large surface area produced by the formationof the nanopores and since the surface energy of P4VP is greater thanthat of PS (γ_(P4VP)=50.0 mJ/m², and γ_(PS)=45.5 mJ/m²). Sohn, B.-H.;Seo, B.-W.; Yoo, S. I.; Zin, W.-C. Langmuir 2002, 18, 10505.Consequently, a recovery to the original morphology would be expectedinitially by heating the films above the T_(g). The thickness of theoriginal and reconstructed films, as measured by optical ellipsometryand x-ray reflectivity, were 24.1 nanometers and 27.1 nanometers,respectively. Upon heating the reconstructed film to 115° C. for 10minutes, the thickness decreased to 24.1 nanometers, and the originalmorphology was recovered. It should be noted that continued heating ofthe film results in a reorientation of the microdomains parallel to thefilm surface, due to preferential interfacial interactions of P4VP withthe substrate and the lower surface energy of the PS block. The movementof P4VP to affect the recovery of the reconstructed films was used tocontrol the spatial placement of material placed on the surface of thereconstructed films Gold was thermally evaporated (at a rate of 0.01nanometer/second) onto the reconstructed film at a glancing angle undera pressure of 5×10⁻⁶ Torr, to a nominal thickness of 1 nanometer, asmeasured by a quartz crystal microbalance. During evaporation, atomicgold diffuses on the surface of the reconstructed film and coalesces,forming nanoparticles. Gold preferentially interacts with the P4VPblock, and a thin layer (about 2 nanometers on average) of denselypacked gold nanoparticles forms on the surface.

FIG. 2 shows TEM images of three different reconstructed PS-b-P4VP filmsdecorated with gold in different ways, depending on the experimentalconditions. The scale bars in the FIG. 2 images are 100 nanometers. FIG.2(a) shows gold nanoparticles located on top of a reconstructed film,where gold sputtering was carried out at a glancing angle (about 5°) tothe substrate so that gold nanoparticles selectively decorated the filmsurface only, without entering into the pores. Consequently, ananoporous gold film is produced that is suitable as a mask for patterntransfer. By heating a gold-decorated reconstructed film having anominal gold layer thickness of less than or equal to 0.5 nanometers(thicknesses of about 0.2 to about 0.5 nanometers have been used) to115° C. for 10 minutes, the gold nanoparticles are drawn into the poresalong with the P4VP, as shown in FIG. 2(b). If the gold layer is thickerthan 0.5 nanometer, this does not occur. It should be noted that evenafter the gold goes into the pore, the size of cylindrical microdomainsdoes not change in comparison to that of the reconstructed film. Whenthe annealing temperature is increased to 180° C. for 30 minutes, thegold-coated reconstructed films undergo a different recovery. If thethickness of the evaporated gold is >0.5 nanometer, some of the gold isdrawn into the pores forming nanoparticles in the center of themicrodomains, leaving the remainder of gold on the surface. Viewed fromabove, a ring pattern forms, as shown in FIG. 2(c), which was generatedusing a gold film with an initial nominal thickness of greater than 0.5nanometers (thicknesses of about 0.6 to about 1 nanometer have beenused). This pattern is suitable for transfer to the underlyingsubstrate. In addition, this process offers a simple but direct means ofplacing nanoparticles at a precise location between two conducting orsemi-conducting layers.

The location of gold nanoparticles on the PS-b-P4VP film was alsoinvestigated by grazing incidence-small angle x-ray scattering (GISAXS).GISAXS measurements were performed at beamline X22B (NationalSynchrotron Light Source, Brookhaven National Laboratory) using x-rayswith a wavelength of λ=1.525 Å. The GISAXS patterns were measured below(α=0.12°) and above (α=0.18°) the critical angle of the polymer(α_(c)=0.16°). The former provides structural information at thesurface, while the latter provides structural information throughout thefilm A cursory examination of the GISAXS patterns is that theinterferences characteristics of the hexagonal array of the microdomainsare extended in the q_(z) direction, that is, normal to the filmsurface. Further assessment of the scattering requires a closerexamination of the profiles. Shown in FIG. 3(a)-(f) are traces takenalong the horizon, that is, along q_(y) (q_(z)=0). For the reconstructedfilm with gold evaporated on the surface, the GISAXS profiles above andbelow the critical angle are identical in shape, though the intensitiesdiffer. For α<α_(c), scattering characteristic of a hexagonal array ofholes in the gold film is seen. For α>α_(c), a significant increase inthe scattering is seen due to an increase in the contrast, that is, theelectron density difference between the air in the pores and the matrix.Hence, an increase in the intensity and a sharpening of the interferencemaxima in the GISAXS, for α>α_(c), are seen in FIGS. 3(a) and 3(b). TheGISAXS results indicate that gold is uniformly distributed in thecylindrical microdomains as one proceeds from the top of the film to thebottom, as shown in FIGS. 3(c) and (d). The gold has been drawn into thecylindrical microdomains but there does not appear to be any significantaggregation of the gold in the pores. When the thickness of the goldis >0.5 nanometer, the scattering arising from the gold layer is seenfor α<α_(c) and α>α_(c) (FIGS. 3(e) & (f)). However, a pronouncedmaximum arising from the form factor of gold nanoparticles within thepore is seen. This scattering is not seen in the other cases, indicatingthat the formation of gold nanoparticles does not occur. It should benoted that scattering from the nanodots is characteristic of a singlelayer at the substrate interface.

To transfer the pattern produced by the gold coated polymer films intothe underlying substrate, the films were exposed to a SF₆ reactive ionetching (RIE; TRION technology, at 50 millitorr pressure, 25 standardcubic centimeters per minute (SCCM) flow rate, and 40 watts power).After etching into the silicon oxide, the gold coated films were removedwith a 10 weight percent potassium iodide/iodine solution (KI/I₂solution; 4/1 volume/volume), followed by oxygen plasma etching for 10minutes. FIG. 4 shows pattern transfer of three gold decorated polymertemplates into silicon oxide using SF₆ RIE, which are identical to thoseseen in the original templates. The scale bars in the FIG. 4 images are100 nanometers. In the inset, pattern transfer results are magnified forclarity. It should be noted that the metal coated films can be used ashard etching masks for pattern transfer into the underlying substrate.

In addition to pattern transfer from gold decorated films, a metalnanoring pattern could also be produced. FIG. 5 shows the SEM image of achromium nanoring pattern obtained from the gold-decorated films havingnanoring-shaped voids, each nanoring-shaped void defined by a circularvoid in the gold layer and a gold nanodot on the substrate surfacedirectly under the circular void. The scale bar in the FIG. 5 image is200 nanometers. The gold-decorated nanoring pattern were reactively ionetched (TRION technology, at 50 millitorr pressure, 30 SCCM flow rate,and 50 W power) with trifluoromethane (CHF₃) to remove polymersurrounding the gold nanoparticles in the cylindrical microdomains.Then, chromium was thermally evaporated onto the etched template to anominal thickness of about 8 nanometers, as measured by a quartz crystalmicrobalance. The films were then sonicated in chloroform for 5 minutesto remove the excess chromium. In the magnified inset in the top-leftcorner of FIG. 5, the chromium nanoring pattern with a long-range orderis clearly seen.

Additional experimental details are provided in S. Park, J.-Y. Wang, B.Kim, and T. P. Russell, “From Nanorings to Nanodots by Patterning withBlock Copolymers”, Nano Letters 2008, 8, 1667, as well as the SupportingInformation accompanying that article.

In summary, we have demonstrated methods for the generation ofmetal-decorated block copolymer films with nanodot and nanoringpatterns. Films of block copolymers having arrays of highly orderedcylindrical microdomains were prepared by solvent annealing. Using apreferential solvent for the minor phase, a reversible reconstruction ofthe films occurred, producing a nanoporous template. By varying theamount of metal evaporated onto the surface, nanoporous metal filmscould be produced and annealed to draw the metal into the cylindricalmicrodomains (forming metal nanodots), or a nanoporous metal film withmetal nanodots at the base of the pores could be produced (therebydefining a nanoring-shaped void in the metal coverage of the substrate).The metal-coated polymer templates could be transferred into thesubstrate with high fidelity using reactive ion etching. And a substratedecorated with metal nanorings could be obtained from further processingof the structure comprising a nanoporous metal film with metal nanodotsat the base of the pores.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope of the inventionis defined by the claims, and may include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral language of the claims.

All cited patents, patent applications, and other references areincorporated herein by reference in their entirety. However, if a termin the present application contradicts or conflicts with a term in theincorporated reference, the term from the present application takesprecedence over the conflicting term from the incorporated reference.

All ranges disclosed herein are inclusive of the endpoints, and theendpoints are independently combinable with each other.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. Further, it should further be noted that the terms “first,”“second,” and the like herein do not denote any order, quantity, orimportance, but rather are used to distinguish one element from another.The modifier “about” used in connection with a quantity is inclusive ofthe stated value and has the meaning dictated by the context (e.g., itincludes the degree of error associated with measurement of theparticular quantity).

The invention claimed is:
 1. A nanopatterned article, comprising: asubstrate; and a metal-decorated block copolymer film in contact with asurface of the substrate; wherein the metal-decorated block copolymerfilm has a thickness and comprises a continuous phase and a dispersedphase comprising a hexagonal array of cylindrical domains extendingthrough the thickness of the film; wherein the hexagonal array has anorientation order of at least 0.9 over an area of at least 1centimeter²; wherein the continuous phase comprises a major phase of theblock copolymer; wherein the dispersed phase comprises a minor phase ofthe block copolymer and nanodots comprising metal; and wherein thenanodots in the dispersed phase constitute at least 50 weight percent ofthe metal content of the article.
 2. A nanopatterned article,comprising: a substrate; a metal-decorated block copolymer film incontact with a surface of the substrate; and a metal layer in contactwith a surface of the metal-decorated block copolymer film opposite thesubstrate; wherein the metal-decorated block copolymer film comprises ablock copolymer and metal nanodots; wherein the metal layer defines anarray of essentially circular voids, each circular void disposed above ananodot; and wherein each circular void is separated by anearest-neighbor distance of 10 to 100 nanometers.